Conveyor apparatus of the kind having an endless belt travelling about drive and guide rollers to effectuate the transport of sheet material have long been known. Rapid conveyor speeds, however, create transfer problems for lightweight sheet materials, such as sheets of paper or cardboard.
A related problem exists in the sheet delivery units of printing machines. In these units, sheets are typically received from the printing press for final disposition at a stacking unit. In order to provide uniform stacking of the sheet material, the delivery units must provide a controlled braking action to effectuate the transition from the high-speed transfer of the printing press to the final, stacked position. Accordingly, various suction means for use with conveyor apparatus have been proposed to solve such transfer and delivery problems.
For example, U.S. Pat. No. 3,389,908 discloses the use of a toothed belt guided in a longitudinally grooved recess in a base plate. In order to reduce the friction on the underside of the toothed belt, the recess is subjected to ambient pressure on the side opposite the teeth. Separately, a vacuum is created on the exposed surface of the belt in the spaces between adjacent teeth, said vacuum being applied by way of longitudinally extending ducts disposed laterally along the sides of the toothed belt.
A disadvantage of this solution is that the sheets are no longer positionally fixed on the toothed belt by the vacuum in the region of the guide roller facing the sheet stack and each sheet has to cover a considerable remaining distance without any appreciable guidance or control. Frequently, this results in uneven delivery of the sheets and stack formation problems.
Another example is shown in DE 3,638,322, disclosing a sheet conveying device having a toothed belt guided in a base member. Suction ducts are formed laterally inside the base member in such a manner as to be open in the region of the tooth gaps and closed in the region of the teeth. A serious shortcoming of this solution, somewhat similar to the problem in the above-mentioned U.S. Pat. No. 3,389,908, is that only the tooth gaps are active due to the change between the open and closed suction ducts and the braking effect is distributed superficially only over these gaps. As a result of the tooth pitch, the flow suction air is periodic which results in considerable fluctuations in the vacuum. Accordingly, there are constant variations in sheet braking and control.